Amide-imide polymers have been widely accepted in the magnet wire and insulation field because of their easy processability and good insulating properties. For example, in addition to providing adequate insulation in an electrical environment, such polymers are known to have good high temperature stability as well. Such polymers are typically used in a two-coat construction as an overcoating on magnet wires coated with cross-linked polyester materials. This provides thermal stability and solvent resistance to the magnet wire not provided by the polyester alone.
Typical synthesis for producing such amide-imide polymers includes the reaction of trimellitic anhydride with 4,4'-diisocyanato-diphenylmethane in a water free solvent system such as N-methyl pyrrolidone and xylene as follows: ##STR1##
This synthesis is taken to a relatively high molecular weight in order to obtain a polymer with the required thermal, chemical and mechanical properties necessary for an acceptable magnet wire enamel. Note U.S. Pat. Nos. 3,541,038; 3,592,789; 3,790,530; 3,843,587; and 4,259,221. Another reason that this synthesis is generally continued to a relatively high molecular weight of polymer is due to the fact that unreacted isocyanate groups will readily react even at low temperatures with any material with an active hydrogen (for example, water) and thus, prevent further molecular weight increase during cure.
However, one problem resulting from the high molecular weight synthesis is that the resulting polyamide-imide enamels have relatively low polymer solids content and relatively high enamel viscosities. Because of the high enamel viscosity and low enamel solids content large amounts of costly organic solvents such as N-methyl pyrrolidone and other hydrocarbon solvents must be employed for magnet wire application.
Another problem resulting from the high molecular weight synthesis is that the high molecular weight polymer also makes complete solvent removal difficult during cure after the polymer is applied to the wire. Trapped solvent in the polymer can cause loss of magnet wire properties and/or a blistered non-smooth coating surface on wire.
Accordingly, if a synthesis method could be found that restricts polymer molecular weight growth to a level that results in an enamel of higher solids content while maintaining an enamel viscosity in a desirable range for application, and at the same time further polymer polymerization and increase in molecular weight could be obtained during cure on the wire thus obtaining desirable magnet wire properties, an improved enamel would result with a much reduced wire manufacturing cost.